1. Field of the Invention
The present invention relates to the cleaning of substrates with supercritical fluids, and, more particularly, to a process employing fluids in the supercritical state, such as carbon dioxide, for cleaning parts without the use of a containment system or pressure vessel.
2. Description of Related Art
Currently, cleaning with a supercritical fluid is performed in pressure vessels which are capable of achieving pressures up to 5,000 psi and temperatures of up to 100.degree. C. This technology provides the conditions required to exceed the critical points of most candidate supercritical fluids (SCFs) such as nitrogen, oxygen, argon, helium, methane, propane, carbon dioxide, and nitrous oxide. The critical points of most fluids useful for precision cleaning are all above atmospheric conditions (i.e., elevated temperature and pressure). Therefore, in order to use these fluids for cleaning, the part to be cleaned must be placed inside a vessel or containment system capable of withstanding the temperatures and pressures required to exceed the fluid's critical point.
Under supercritical conditions, the gas has the ability to dissolve contaminating species, such as organic molecules. This is to be contrasted with the use of physical removal processes, such as blowing CO.sub.2 "snow" on the part to be cleaned.
Systems based on this technology typically include a pressure vessel, a fluid pump, a fluid reservoir, a separator and condenser system, and various valves, transducers, and temperature sensors. Systems of this nature are expensive and typically cost between $100,000 to $400,000 in 1992 dollars.
The pressure restriction limits the maximum size of parts that could be processed and requires a batch-type operating mode in which the process vessel is loaded, sealed, pressurized and the supercritical fluid is recirculated, depressurized and unloaded sequentially. Such batch mode processing also requires substantial periods of time, particularly in the pressurizing and depressurizing operations.
Thus, a need exists for an apparatus and method of cleaning large or irregularly-shaped parts without having to resort to high pressure vessels and batch-mode processing.